1. Field of the Invention
The present invention relates to an imaging toner and, more particularly, to an imaging color toner suited for use in electrophotographic and other imaging processes employing a photofixing system. The color toner of the present invention can be used advantageously as a developing agent in various imaging apparatuses employing an electrophotographic system and other systems such as ionographic system, for example, electrophotographic copying machine, electrophotographic facsimile, electrophotographic printer and electrostatic printing machine. The present invention also relates to a color image forming method and a color image forming apparatus, which employ the imaging color toner.
2. Description of Related Art
The electrophotographic system which has been widely used in copying machines, printers and printing machines generally begins by charging the surface of a photoconductive insulator such as a photosensitive drum uniformly with a positive or negative electrostatic charge. After charging uniformly, the electrostatic charge on the insulating material is partially erased by irradiating the photoconductive insulator with image light by various means to thereby form an electrostatic latent image. For example, an electrostatic latent image corresponding to image information can be formed on the photoconductive insulator by erasing the surface charge from particular portions by irradiating with a laser beam. Then a fine powder of developing agent referred to as toner is caused to deposit on the latent image where the electrostatic charge remains on the photoconductive insulator, thereby to visualize the latent image. Last, in order to print the toner image obtained as described above, it is common to electrostatically transfer the image onto a recording medium such as recording paper. For the fixation of the transferred toner image, such methods as a fixing method wherein toner is melted by pressurization, heating or a combination thereof and is then solidified, or a fixing method wherein toner is melted by irradiating with light and is then solidified, have been employed, while much interest is being directed to a method referred to as a photofixing method (also referred to as a flash fixing method) which uses light that is free from adverse effects of pressurization and heating. In the photofixing method, since it is not necessary to apply a pressure to the toner during fixation, the need to bring the toner into contact (pressurization) with a fixing roller or the like is eliminated, and such an advantage is provided as imaging resolution (reproducibility) experiences less degradation in the fixing step. Also because it is not necessary to heat the toner with a heat source, the idle time before printing can be started after turning on the power to preheat the heating medium (fixing roller or the like) to a predetermined temperature is eliminated, so that printing can be started immediately upon turning on the power. Eliminating the need for a high temperature heat source has another advantage that the temperature in the apparatus does not rise too high. It also eliminates such a danger that the recording paper catches fire due to the heat generated by the heat source, even when the recording paper jams in the fixing device due to a system breakdown or other trouble.
However, a color toner has a low efficiency of absorbing light and, when the photofixing method is applied to the fixation of color toner, the fixability is lower than that in the case of fixing a black toner. Thus it has been proposed to improve the fixability by adding an infrared absorbing agent to the color toner, and many patent applications related to this technology have been laid-open as: Japanese Unexamined Patent Publication (Kokai) Nos. 60-63545, 60-63546, 60-57858, 60-57857, 58-102248, 50-102247, 60-131544, 60-133460 and 61-132959, WO99/13382, and Japanese Unexamined Patent Publication (Kokai) Nos. 2000-147824, 7-191492, 2000-155439, 6-348056, 10-39535, 2000-35689, 11-38666, 11-125930, 11-125928, 11-125929 and 11-65167. Technologies disclosed in these publications are attempts at making color rendering and photofixability compatible with each other by adding an infrared absorbing agent to the toner. However, all of the infrared absorbing agents proposed have the problem of inability to achieve satisfactory fixation.
Moreover, the photofixing method has such a problem that the air in the vicinity of toner expands and the toner boils resulting in voids (whiting defects) which are minute printing defects, because the toner is subjected to instantaneous heating by the irradiation with flashlight in the photofixing method. To address this problem, Japanese Unexamined Patent Publication (Kokai) No. 5-107805 discloses an attempt to prevent voids from occurring by controlling the softening point, the glass transition point and the acid value of a binder resin which is used as a major component of toner. However, this method is not effective in preventing voids from occurring when a color toner is used. In the case of a color toner, the binder resin is designed to have lower viscosity than that of the case of black toner for the purpose of smoothing the toner image, which leads to the generation of voids.
Moreover, Japanese Unexamined Patent Publication (Kokai) No. 4-338973 proposes to use a mixture of two kinds of binder resins as the binder resin. However, this method cannot make flash fixability and void resistance compatible with each other.